Portable electronic devices such as personal digital assistants (PDAs), wireless telephones, and the like typically have rechargeable batteries for powering the devices. Because such devices from time to time temporarily require the provision of higher than the normal current supplied by the main battery, e.g., during certain periods of wireless transmission of data, supplemental batteries might be provided to make up the difference.
For such systems, the supplemental batteries typically are recharged using the main battery during periods of low current demand. This presents challenges, particularly in the case of Lithium ion (Li-ion) supplemental batteries which are increasingly popular due to the fact that they can be made very thin and, hence, supportable on an integrated circuit (IC) card within the portable electronic device. With more specificity, Li-ion batteries require a constant charging current until a charge voltage (e.g., 4.2 volts), is reached, at which time it is desirable to provide a constant voltage to the battery to complete the charge. But when the main battery is at a low voltage, providing this kind of charging profile can require the use of a relatively large boost regulator so that the charge current to the supplemental battery will not exceed its design limit. This consumes valuable space in what is designed to be a small electronic device, and moreover represents a relatively high added cost. The alternative is to use a wall charger, which can be impractical to use under many circumstances. There is thus a need to provide a circuit, which is internal to a portable electronic device, to limit charge current to a supplemental battery, without requiring the use of a relatively bulky boost regulator.